The invention concerns a component with high heat load-carrying capacity, comprising at least one graphite part with at least, which parts are connected to each other by a layer of solder, wherein an intermediate layer is arranged between the metal part and the solder.
Because of graphite's special characteristics such as high heat load-carrying capacity, good heat conductivity and low rate of disintegration, it is very suitable for components which are subjected to high thermal stress. In these applications the graphite is used in a variety of forms, such as a polycrystalline graphite, pyrolitic graphite, or also as fiber-reinforced graphite. It is a disadvantage of graphite that, even in its fiber-reinforced form, it features only limited mechanical strength and ductility. For this reason graphite is often used only as a composite material in connection with metals, in order to fulfill all the requirements for that special application.
The soldering of graphite to suitable metallic materials--such as for instance copper, inconel, molybdenum or TZM--is known. Examples of suitable solders are, for instance, alloys of the elements Ag, Cu, Ni, Pt, Ti, Zr, Cr with a proportion of copper and titanium. The soldering is carried out in a vacuum, at temperatures which, depending on the solder, range between 850.degree. C. and 1900.degree. C.
Graphite/metal composite materials have proved their advantages in actively cooled heat shields, such as for instance in the production of components for the "first wall" or of diverters and limiters in the case of fusion reactors. In those applications the graphite part is soldered to cooling pipes such as those made of copper or molybdenum.
One of the problems in the case of all soldered composite materials made of graphite and metals is the great difference in the coefficient of thermal expansion, a difference which causes enormous stresses on the soldered connection between the different materials. Particularly in the case of cyclical thermal stresses, the soldered connection often does not resist the varying mechanical stresses that occur in such cases, so that major distortions, cracks in the material or even a complete failure of the component may occur.
Consequently, the past has been punctuated by numerous attempts to provide better matching between the differing thermal expansions of graphite and metals as used in soldered composite materials, or to reduce the mechanical stresses that occur, by inserting an intermediate layer.
For instance, JP 63-310778 describes a composite material produced by soldering, designed for high-temperature applications and made of graphite and metal, in which a single-layer intermediate layer of nickel is provided in order to reduce the mechanical stresses in the composite material.
AT-B-393 651 describes a composite body resistant to high temperatures made of a graphite part and a part consisting of molybdenum or a molybdenum alloy, the parts being connected to each other by a zirconium solder, wherein a two-layer intermediate layer made of vanadium and tungsten or tantalum or niobium is arranged between the molybdenum part and the solder. The intermediate layer is designed to prevent the formation of a eutectic between molybdenum and the zirconium solder, which eutectic would lower the maximum permissible usage temperature of the composite body.
DE-A1-2527326 describes a switching part for high-voltage power switches, in which the surface of the switching part, made of a high-melting metal, is connected to an electric-arc electrode made of graphite, via a layer of solder containing at least one carbide-forming metal. As an option one can arrange an intermediate layer made of a carbide-forming metal such as chromium, between the graphite electrode and the solder layer. In this manner, the wetting of the graphite--hence the strength of the solder connection--is to be improved. Over and above such an improvement, such an intermediate layer should go far to prevent an undesirable diffusion of carbon from the graphite into the solder material.
EP 0 296 942 also describes such a composite material for high-temperature applications in which there is arranged, between the parts to be connected, a foil made of a high-melting metal such as Mo, Ta, Hf, Zr, Nb or W and their alloys; as well as a foil made of Cu, Ti or Ni and their alloys. The soldering material is arranged between the parts to be connected and the foils, as well as between the two foils themselves. This type of connection yields components which provide for a good connection of the graphite and metal materials, in the case of uniform heat stresses. However, in the case of sharply cyclical thermal stresses--such as occur, for instance, in cooling elements for fusion reactors--this type of connection is not an optimum one and may lead to stress cracking in the material and/or to failure of the entire components.